On a hot summer day, Holmes and Dr. Watson went on a camping trip. After a good meal and a bottle of wine, they lay down in a tent for the night and went to sleep.
   Some hours later, Holmes woke up and pushed his friend. “Watson, look! What can you see in the sky?"
   Watson replied, “I see millions and millions of stars.”
   “What does that tell you?” asked Holmes.
   Watson thought for a while. “First, it tells me that we will have a beautiful clear day tomorrow. Second, it tells me that the time is about twelve o’clock. And…”
   “What else?” Holmes asked.
   “Well, it also reminds me that the world is so big and we are so small. What does that tell you?” Holmes said with a grin on his face, “You silly guy! It only tells me that someone has stolen our tent!”

一个木箱在水平拉力F的作用下沿光滑水平地面滑动，有四位同学作出它的受力情况如图所示，其中正确的是（　　）

 在探究小鼠走迷宫获取食物的实验中，某组同学的设计方案如右图所示：在迷宫的出口处放一些小白鼠喜爱的食物，将小白鼠放进迷宫入口，记录小白鼠“尝试与错误”次数．据此回答：

请你来当邮递员连一连.

下列做法可以达到预期目的是（　　）